Neutral heating with controlled preheat



July 29, 1958 v F. A. RUSCIANO 2,845,260

- NEUTRAL HEATING WITH CONTROLLED PREHEAT Filed April 9, 1954 4Sheets-Sheet 4 INVENTORQ F.A.RUVS-C|ANO4 ATTORNEY United States PatentNEUTRAL HEATING WITH CONTROLLED PREHEAT Frank A. Rusciano, New York, N.Y., assignor to Metallurgical Processes (30., Newark, N. J., acorporation of New Jersey Application April 9, 1954, Serial No. 422,074

17 Claims. c1. 266-5) This invention relates to a method for producingrapid and efficient heating of metals to elevated temperatures undersubstantially non-scaling conditions and to a fur nace structure bywhich such method may be practiced.

In an application Ser. No. 347,716, filed April 9, 1953, and entitledMethod and Apparatus for Producing Controlled Furnace Atmospheres amethod is disclosed for producing a non-scaling atmosphere in a directfired furnace in which the metal being heated is subjected directly tothe products of the furnace burners. Briefly, this method comprisesadjusting the air-fuel ratio of the burners so that the reactionproducts, when the reactions are carried to substantial completion, willhave a 00 /00 and Ego/H ratio of such value that the sum thereof isequal to unity. The reactions resulting from such mixture, whileproviding an ideal non-oxidizing atmosphere for the furnace, release arelatively small percentage of the available B. t. u. of the fuel, ofthe order of 25%, and therefore provide a low heating rate and arestricted upper temperature limit. In the process of the aforesaidapplication, these limitations are overcome by the subsequent combustionof this atmosphere gas externally of the work chamber and out of contactwith the work but in heat transfer relation thereto.

The present invention utilizes the principal features of the aforesaidapplication, one of its objects being to increase the heating rate overthat obtainable with the previous process.

Another object is to increase the operating efliciency of the aforesaidprocess and to decrease the cost of the furnace equipment employed inthe practice thereof.

A still further object is to provide a novel furnace structure for theproduction of scale-free or substantially scale-free heating of metals.

Other objects and advantages will appear as the description proceeds.

In accordance with the present invention the heating of the metal atelevated temperatures, that is, from a temperature in the range fromabout 1000 F. to 1500 R, up to the desired final temperature, which maybe as high as 2500 -F., is carried out in the neutral combustionatmosphere disclosed in the aforesaid application for patent, and therich neutral atmosphere gas so employed is subsequently combusted withadditional air in a combustion chamber disposed externally of the hightemperature heating zone but in heat transfer relation thereto, asdisclosed in said application. The heating rate and heating efficiencyof this arrangement is somewhat less than that of a direct fired furnacein which the fuel is combusted to substantial completion in the workheating chamber due to the losses inherent in the conduction of heatfrom the external combustion chamber to the work treating chamber. Thepresent invention eliminates this efliciency differential over a largeportion of the heating cycle by conducting the initial heating at themaximum permissible non-scaling rate up to the 1000 F. to 1500 F. range,in a substantially completely combusted atmosphere, that is, acombustion atice mosphere in which the COg/CO ratio approaches infinity.In this initial heating operation the temperature of the initial heatzone should be controlled so as to prevent the surface of the metal fromattaining a temperature appreciably above the recited range for anysubstantial period. In other words, a balance should be maintainedbetween the rate of heat absorption by the surface of the work and therate of heat diffusion into the body of the work whereby the surfacetemperature will be retained substantially at but not above that atwhich material scaling occurs. When this balance is just maintained, forinstance, by control of the quantity of combustible mixture supplied tothe initial heating chamber, the preheating will be effected at themaximum permissible non-scaling rate.

In many heating operations, as for instance, in the heating of billets,bar stock and other heavy pieces for mechanical working, suchas'rolling, piercing, swaging, forging, or the like, a light surfaceoxidation or slight scale may be entirely unobjectionable, or evendesired, and in such cases the initial heating in the oxidizingcombustion atmosphere, if not prolonged, may be carried up to surfacetemperatures of about 1500 F. or, in the case of some alloy steels, upto as high as 1700 F. Thus, the invention contemplates an initialheating of metal up to a predetermined surface temperature in the rangebetween 1000 F. and 1700 F. in a combustion atmosphere which at highersurface temperatures would be highly oxidizing to the metal, followed bya heating to final temperature in either a non-scaling or a reducingatmosphere, the initial heating range depending upon the nature ofthemetal, the rate of heating and the amount of allowable oxidation of thework when it attains its final temperature. Above this initial heatrange it is necessary, in order to prevent further scaling, to decreasethe oxygen contentof the air-fuel mixture to approximately 50% of thatrequired forcomplete combustion so as to produce an atmosphere in whichthe ratio of CO to CO will be in the range from about 0.3,to 0.6,depending on the temperature, and in which the sum of the 00 /00 and HO/ H ratios will be equal substantially to unity. If it is desired toreduce some of the scale produced in the initial heating, it isnecessary to use somewhat less air in the mixture employed to producethe final heating atmosphere,

The invention is described herein with particular reference to acontinuous furnace but it is equally applicable to other types 'offurnaces. In the present embodiment the furnace comprises a linear orstraight-through heating chamber in which the work to be heated passesfrom one end to the other. This chamber is divided into three heatingzones, the first extending from the charging opening to a point at whichthe work will have attained,. in a prescribed time, a temperature in the1000 to1700 F. range, and the second and third zones extending in.succession from the end of the first zone to the discharge end of thechamber. The first zone is provided with a high temperature atmosphereobtained by the substantially complete combustion of a fuel and airmixture in burners extending into this zone, although the combustioncould equally well be effected in separate combustion chambers in.direct communication with this zone. The second zone is in continuousfree communication with the first .zone and the third zone is in similarrelation to the second zone, one merging into the other-and forming partof the same heating chamber. The second and third zones are eachprovided with a hot atmosphere produced by the reaction or combustion ofa fuel and air mixture having a sutficient deficiency of air requiredfor complete combustion to render the atmosphere non-scalingto, the workat the elevated temperature attained in these zones. This non-scalingatmosphere gas is also generated in 3 burners directly associated withthe heating zone. Since the air-gas mixture here employed is at thelower end of the exothermic range, it is desirable to add somesupplemental heat to, the mixture in order to insure completion of thereactionsprior to contact of the reaction products with the work.Various means for supplying this supplemental heat may be employed, asdisclosed in the aforesaid applicatiorn-but in the present embodiment itis obtained by employing the exhaust gases from the furnace to heat theair supplied to the burners of the two final heating zones.

The second heating zone serves to heat the metal from the preheat rangeto an intermediate temperature, for example, 2000 E, which is stillconsiderably below the final work temperature desired, say 2350" F. Thetemperature head required in the second or intermediate temperature at aprescribed rate is, of course, lower than the temperature head requiredto heat the metal from the intermediate to the final temperature at thesame heating rate. It has been found with a given total quantity of heatrelease in the second and third zones that a considerably faster overallheating rate in those zones can be obtained by dividing this total heatrelease in such a way as to produce a materially higher temperature headin the final zone over that maintained in the intermediate zone, and theinvention contemplates effecting this result in a manner which willmaintain a neutral atmosphere in both zones.

The neutral or non-scaling atmosphere produced in the second and thirdzones, because of its large deficiency in oxygen, produces a relativelysmall amount of heat, approximately 20% to 30% of that produced in theinitial zone, per unit of fuel. This atmosphere therefore contains anadditional 70% to 80% of available heat. This potential heat isconverted into sensible heat by completing the combustion thereof withadditional air in a second combustion chamber separated from the workheating chamber by a relatively thin partition of high heatconductivity.

The invention further includes means for segregating the atmospheresproduced in the initial and intermediate heating zones and in coolingthe exhaust gas from the initial zone, when necessary, to a suitabletemperature for passage through an alloy heat exchanger, as will fullyappear from the description of the accompanying drawings, in which:

Fig. 1 is a central vertical longitudinal section of a furnace embodyingthe features of the invention;

Fig. 2 is a vertical transverse section of the furnace taken on the line22 of Fig. 1; e V

Fig. 3 is a vertical transverse section taken on the lin 3-3 of Fig. 1;

Fig. 4 is a vertical transverse section taken on the line 4-4 of Fig. 1;

Fig. 5 is a detailed sectional view of an air inlet nozzle employed forthe secondary combustion chamber;

Fig. 6 is a detailed sectional view of one type of burner employed inthe furnace shown;

Fig. 7 is a fragmentary sectional view taken on the line 7--7 of Fig. 1;and

Fig. 8 is a schematic view of the piping and valving arrangement for thesupply of air and fuel to the furnace.

Reference will first be made to Figs. 1 and 2 in which a continuousfurnace of the gravity or roll-down conveyor type is shown. The furnaceis composed of refractory brickwork and includes an inclined floor 10,side walls 11 and 12, an arched roof 13 and end walls 15 and 16. The endwall 15 is provided with a restricted passageway forming a charging slot17 and end wall 16 has a similar restricted passageway forming adischarge slot 18, the work W, shown as round bar stock, being conveyedfrom the charging slot to the discharge slot by gravity, rolling uponthe spaced rails 19, composed of a material, such as silicon carbide,which .is capable of withstanding both the load and the temperature. Inthe second and final case of fiat stock, suitable pusher mechanism maybe employed to force the work along the rails 19. Other forms ofconveyance for the work may be provided, such as a belt or chainconveyor, depending on the nature of the work and the temperature towhich it is to be heated. The conveyor mechanism forms no part of thepresent invention, except that it is necessary to provide someconvenient means for transporting the work through the heating chamber.As shown in Fig. l, the bars come to rest against a shouldered portion22 of the rails 19 and are raised over this shoulder for dischargethrough the slot 18, by pneumatically operated levers 23.

The charging and discharging slots or passageways 17 and 18 are providedwith doors 20 and 21, respectively, adapted to be opened and closed byconventional mecha nism, not shown.

A pair of depending bulkheads or partitions 24 and 25 depend from thearched roof 13, bulkhead 24, terminating below the centerline 26 of theburners which extend through the side Walls 11 and 12, and above thefloor or hearth 10 a distance only sufficient for the convenient passageof work therebeneath, so as to produce a restricted passage. Arelatively thick supplemental arch 27 (Fig. 2) extends completely acrossthe heating chamber between the bulkhead 24 and the front end wall 15,and a pair of relatively thin arched slabs 28, 29 (Figs. 1 and 3) extendcompletely across the heating chamber between the bulkheads 24, 25, andbetween bulkhead 25 and rear end wall 16, respectively. Thesesupplemental arches are spaced from the main roof arch 13 to form thechambers 31, 32 and 33, the purpose of which will hereinafter appear.The supplemental arch 27 is composed of a good heat insulatingrefractory, whereas arches 28 and 29 are composed of a material havinggood heat couductivity and high temperature strength, such as siliconcarbide.

The initial heat zone, indicated at 34, extends from the front end wall15 to the bulkhead 24 and is provided, along the burner line 26, at eachside of the furnace with a row of burners 35, those indicated by circlesin Fig. 1 extending into the zone 34 through the side wall 11, and thoseindicated by crosses extending through the opposite side wall 12. Theburners 35, as will more fully appear, are supplied with a mixture offuel and air proportioned for substantially complete combustion of thefuel.

The intermediate heat zone 36 extends from the bulkhead 24 to thebulkhead 25 and is provided with a series of burners 37 extendingthrough the side walls 11 and 12.

The final or high temperature heat zone 36 extends from the bulkhead 25to the discharge slot 18 and is provided with a series of burners 38extending through the side walls 11 and 12. Burners 37 and 38 aresupplied separately with very rich mixtures of fuel and air proportionedsomewhat dilferently, as will later appear, but of such ratio as toproduce a non-scaling atmosphere in each of the zones 36 and 36'. Themixture, which will produce a combustion atmosphere which is justneutral, as stated, is one which on complete reaction will produce atatmosphere in which the sum of the COz/CO and H O/H ratios isapproximately one. The actual CO /CO and H O/H ratios will vary withtemperature, the respective values at 1500" F. both being about 0.5; at1800 F. about 3.8 and 6.2; and at 2100 F. about 0.3 and 0.7. The properair-fuel mixture for obtaining these ratios is largely independent ofthe operating temperature and is controlled primarily by the ratio ofmolecular carbon to hydrogen in the fuel. Thus a fuel having a C/Hmolecular ratio of 0.75, such as propane (C H will be restricted toapproximately 54% of the oxygen required for complete combustion inorder to achieve the above CO /CO and H O/H ratios. With higher C/Hratios, larger proportions of air may be employed. Substantially allindustrial furnace fuels will have a C/H ratio between 0.5 and 1.5 andwill produce the desired COz/CO and H O/H ratios with an air deficiencyof from 40% to 50%. Curves showing these relationships are disclosed inthe aforesaid application.

The initial heat zone 34 is vented by means of a series of flues 39extending from the floor level upwardly through each of the side walls11 and 12 and terminating in the upper arch chamber 31. Zones 36 and 36are likewise vented to the chambers 32 and 33 by means of flues 40 and41, respectively. Each of the flues 39, 40 and 41 is provided at itsupper end with a nozzle, such as 42 (Fig. 5), in proper relationship tothe flue outlet to produce a suction eifect in the flue for withdrawingthe atmosphere gases from the heating chamber under controlled draft andthus at any desired rate depending on the velocity of the air sosupplied and the adjustment of the nozzles relative to the ports. Thenozzles provided for chamber 31 are designated 42a and those associatedwith chambers 32 and 33 are designated 42b and 42c, respectively.Referring to Fig. 5, the nozzle 42 is shown as having a conical endwhich may be adjusted relative to the port 43 extending into the chamber31, 32 or 33, so as to regulate the effective port area. This nozzle iscomposed of a high temperature refractory, such as silicon carbide, andextends into a refractory block 44 of similar material, forming the flueoutlet port 43. The nozzle is carried by an alloy metal cup 45 welded toan externally threaded pipe 46. A tube 47, welded to the shell of thefurnace,

carries a pipe flange 48 to which a plate 49 is bolted. The plate 49 hasa central aperture in threaded engagement with the pipe 46, and thelatter is slotted at 50 to receive a tool for turning the pipe to adjustthe nozzle relative to the port in block 44. The outer end of the pipe46 is enclosed in a tube 51, welded at one end to plate 49 and having aT connection 52 at the opposite end with the air conduit 53. A removableplug 54 permits access to the pipe 46 for the adjustment thereof.

The function of the nozzles 42a provided for the flue 39 is two-fold.First, they create the desired suction in the flues to control thepressure in zone 34, and secondly, they supply air to the chamber 31 tocool the exhaust gases to a temperature of approximately 2000 P. so thatthese gases may be passed through a prefabricated alloy heat exchanger60 carried on the roof of the furnace. The heat exchanger is open to thechamber 31 by a group of passageways 61 and to the outer air by anoutlet opening 62. A series of thin wall alloy tubes 63 are containedwithin the heat exchanger, extending between the headers 64and 65. Coldair is admitted to header 64 by a conduit 66 and heated air is conductedfrom header 65 by one or more conduits, such as 67.

The burners employed for zone 34 each comprise a burner block 71 of highheat-resisting refractory, such as silicon carbide, and a nozzle 72, airand gas, at suitable pressure, being supplied to the burner nozzle by aconduit 73 so as to admit the desired quantity of gas and air premixedin completely combustible proportions.

The burners 37 and 38 employed for zones 36 and 36', respectively, areshown in Fig. 6. They comprise an elongated burner block 71' ofheat-resisting refractoryhaving a recess 74 and an intermediaterestriction 73'. Air under suitable pressure and at an elevatedtemperature is supplied to the burner by a nozzle 74 and gas at suitablepressure is supplied to the burner recess by a conduit 75. The relativequantities of air and gas are so regulated, as will later appear, toproduce a nonoxidizing atmosphere in the zones 36 and 36'.

The air nozzles 42b and 42c provided for the flues and 41 serve the dualpurpose of controlling the pressure in zones 36 and 36' or" the heatingchamber and to supply air to the gases exhausted therefrom for thepurpose of completing the combustion thereof in the arch chambers 32 and33. Y

Each of the chambers 32 and 33 are vented by flues 76 and 77,respectively, which terminate above the furnace roof. These gasesdischarge into a wedge-shaped chamber 78, formed in a body of refractorybrickwork 79, the narrow or restricted upper end of which is connectedto the inlet of a heat exchanger 60 similar to heat exchanger 60. Thechamber 78 is open to the air by passageways 80 in the base of thebrickwork for the purpose of inducing cooling air into the exhaust gasstream to reduce the temperature thereof to enable these gases to besafely passed through the metal heat exchanger. Baflles 81 disposedopposite the passageways 80 are adjustable to control the amount of airso admitted. Cold air under pressure is admitted by conduit 66' to oneend of the heat exchanger and heated air is extracted from the oppositeend by conduit 67.

The end walls 15 and 16 through which the restricted work loading anddischarge passageways 17 and 18 extend are provided respectively withvents 39 and 41', each of which is also supplied with a suctionproducing nozzle, designated 42d and 42e, respectively, The particularpurpose of these vents is to create a zone of reduced pressure in theslots so as to reduce the flaming out of the furnace gases when thedoors 20 or 21 are opened. This is particularly important at thedischarge end since the atmosphere in the final heat zone 36, because ofits high oxygen deficiency, will burn vigorously if it is permitted tovent throughthe door opening. The zone of reduced pressure formed in theslot 18 acts to restrict this venting when the door is open and furtherprevents burning around the edges of the door when it is closed. Thesuction effect of nozzles 42d and 42e may be adjusted, if desired, tocreate an inward flow of air as distinguished from an outward flow offurnace atmosphere gases.

Reference will now be had to Fig. 8 for a disclosure of the valving andpiping arrangement of the furnace. The air passed through the heatexchangers 60 and 60 under suitable pressure is supplied to the inletconduits 66 and 66' by a pump or blower P and the size of these heatexchangers is assumed to be sufficient to heat this air to a temperaturebetween 500 F. and 1000 F. This heated air is supplied by conduits 67and 67 to a manifold 82 and fed therefrom to the conduits 83 and 84leading to the manifolds 85 and 86 for supplying pressurized heated airto the nozzles 74 of burners 37 and 38 of the intermediate and finalheat zones 36 and 36', respectively. Conduits 87 and 87 also convey thisheated air to nozzles 42b and 420 of arch chambers 32 and 33 by way ofmanifolds 88 and 88. Nozzles 42a of arch chamber 31 are supplied withunheated pressurized air by conduit 89 and manifold 90. Unheated air isalso supplied to the burners 35 of the initial heat zone 34 by amanifold 91 fed from a conduit 92 containing a venturi air-gas mixer 93by which gas from a zero pressure line 94 is induced into the air streamin such proportion as to be completely combustible at the burners 35.Conduits 83, 84, 87, 87 and 92 are provided with normally open electricvalves 95a, 95b, 95c, 95d and 952, respectively, by-passed by oriflcedconduits 96a, 96b, 96c, 96d and 96e, respectively. Conduits 87, 87' and92 are also provided with a second normally open electric valve 97c,97d, 97e, respectively. Conduit 89 has a rotary valve 100 thereinoperated to open and closed position by a two-directional motor 101 incircuit with contacts 102, 103 of a differential pressure regulator 104having pressure tubes 105 and 106 extending from opposite sides of aspring-biased diaphragm to the zones 34 and 36, respectively, of theheating chamber, at points adjacent to the bulkhead 24. The purpose ofthis differential pressure regulator will appear hereinafter.

Electric valve 95e in the air conduit for the oxidizing burners 35 iscontrolled by a radiation type thermocouple 107 (Fig. 7) which sightsdirectly upon the Work W at a point in zone 34 adjacent to the bulkhead24 and serves to actuate the contacts 108, to close valve 952 wheneverthe surface of the Work in the initial heat zone attains its prescribedmaximum temperature in this zone, thereby placing the burners 35 on areduced air and fuel supply, as determined by the orifice in the by-passline 96a.

Electric valves 95a to 95d in the air lines 83, 84, 87 and 87 and valves95 and 95g in the fuel lines 98 and 99 of burners 37 and 38,respectively, are controlled by a pyrometer 109 in the final heat zone36, when the prescribed maximum temperature is attained in this zone,thus placing burners 37 and 38 on low fire and simultaneously reducingthe air supply to the secondary combustion air nozzles 42b and 420proportionally. Fuel valves 95 and 95g are bypassed by orificed lines 96and 96g, respectively.

Each of the valves 95a, 95b, 95c, 95d, 95f, 95g, and 970 to 972 may beclosed by operation of a manual switch 111 to the left, for purposeswhich will subsequently appear. All of these valves close completelywhen energized with the exception of valves 97c and 97d which areadjusted so as to only partially close to thereby permit a low flow ofair therethrough of a value somewhat less than that passed by by-passes96c and 96d when the valves 95c and 95d are closed. The purpose of thearrangement will later be described.

As previously indicated, the venturi mixer 93 is adjusted to supply anair-fuel mixture to the burners 35 of such proportion as to enable thefuel to burn to substantial completion, that is, with a negligibleamount of carbon monoxide and hydrogen in the products of combustion.This mixture produces, in zone 34, a high flame temperature andcombustion products which at temperatures above the rang from 1000" F.to 1700 F. would be highly scaling to steel. It therefore produces azone of high heating capacity in which the initial heating of the workmay be eifected at the maximum non-scaling rate and at high efficiency.As previously stated, some scaling may occur within the specifiedtemperature range but the rate of scaling is relatively slow and bysuitably controlling the supply of fuel to this zone by the surfacetemperature reading pyrometer 107 so as to maintain this surface at themaximum safe temperature, the work may be brought up to the preheattemperature in the minimum period consistent with the restriction of theoxidation to a light scale. formation. Where scale-free work is desired,the initial heat zone may be restricted to temperatures at the lower endof the recited range, although the subsequent heating in theintermediate zone in an atmosphere which is slightly reducing, as willpresently appear, will eliminate any light scale formed in the initialheat zone. For many operations, for instance, in the rolling of billets,a slight scale is desired and for such purposes the time and temperatureof heating in the initial zone can be adjusted to attain the desiredscale.

The valves 95a and 95 for supplying air and fuel to burners 37 of theintermediate zone are adjusted so as to supply an air-fuel mixture tothese burners which will produce reaction products having a high CO andH content, sufiicient to give the unity summation of the CO /CO and HO/H ratios or lower. This mixture is not highly reactive and requiressupplemental energy to carry the reactions to completion, thereby toeliminate soot formation and to produce the desired neutral ratios ofthe reaction products. This energy is supplied by the heated air whichin addition to carrying the reactions to completion in the burner tunnelalso increases the temperature of the reaction products. Thus, byproviding air at a temperature between 500 F. and 1000 F. thetemperature of the gases entering the heating chamber is correspondinglyincreased thereby augmenting the heating effect of the gaseous productson the work. This is an important consideration since the heat producedin the reactions themselves is relatively low in comparison with .hatgenerated by complete combustion of the fuel and an increase of a fewhundred degrees in the temperature of this atmosphere represents afairly large proportionate increase in its heating effect. The valves96b and 96g for supplying air and fuel to the burners 38 of the finalheat zone may also be adjusted so as to supply an air-fuel mixture tothe burners 38 which will produce neutral or nonscaling reactionproducts. Thus burners 37 and 38 may both be supplied with a mixturewhich is just sufiiciently deficient in oxygen to produce neutralreaction products. However, I have discovered that if the air-fuel ratiosupplied to the final heat Zone is increased slightly and the ratiosupplied to the intermediate zone correspondingly decreased, whilemaintaining the total air and fuel supplied to the combined zones at theneutral ratio, a considerably higher heat rate in the combined zones maybe obtained or the same heating rate maintained with a lower fuelconsumption. Thus, if the neutral air-gas ratio for the fuel employed,for instance, natural gas, is approximately 5-1, representing a airdeficiency, then burners 37 of zone 36 may be operated with a slightlylower ratio having an air deficiency of between 50% and say 4.5l, andburners 38 of zone 36 With a higher ratio having an air deficiency ofbetween 40% and 50%, say 5.51. The 5.5-1 ratio would produce reactionproducts having a CO /CO and a H O/H ratio with a summation in excess ofunity and such products would be slightly oxidizing. On the other'hand,the 4.5-1 ratio would produce reaction products having a CO /CO and HO/H ratio summation of less than unity. Therefore, by passing a portionof the richer atmosphere from zone 36 into zone 36' for reaction withthe burner products in zone 36, an atmosphere may be produced thereinwhich will have the neutral or unit ratio summation. The heat generatedin zone 36 by the lower air-gas ratio is substantially less than the 25%obtainable from a 5-1 or neutral ratio. However, the work temperature tobe obtained in this intermediate zone is also low compared to the finaltemperature, and it is comparatively easy by use of heated air and byburning a portion of the rich reaction products to completion in theoverarch chambers 32 to obtain a sufficiently high temperaturedifferential in this zone to effect rapid heating of the work throughits intermediate temperature range.

In zone 36', however, the work must attain its final temperature of,say, 2350 F., and in order that this be effected at a rapid rate it isnecessary that a temperature be obtained in this zone several hundreddegrees higher than in the intermediate zone. This higher temperature isobtained partly by the higher flame temperature obtainable with thehigher than neutral air-gas ratio and partly by the secondary combustionin chamber 33 of the reaction products of burners 38 and thoseintroduced into zone 36 from zone 36. Actual field tests have shown thatthe use of an air-gas ratio less than the neutral ratio in theintermediate zone with a proportionately greater than neutral ratio inthe final zone increases the overall heating rate in the order of 10%and permits higher final work temperatures to be obtained.

Referring again to Fig. 8, the air line valves a to 95d will supply anaggregate amount of air sufficient to burn to completion the totalamount of gas supplied by valves 95 and 95g. Approximately 50% of thisair will be supplied by valves 95a and 95b in the lines to burners 37and 38 and 50% by valves 95c and 95d which supplies the air forsecondary combustion through nozzles 42b and 420. When valves 95a to 95,95f and 95g are closed under control of the pyrometer 109 uponattainment of a predetermined maximum temperature in zone 36', burners37 and 38 are supplied with a proportionately reduced amount of air andfuel by the by-passes 96a and 96b. A proportional reduction in the airsupplied to the nozzles 42!) and 42c is obtained through by-passes 96cand 96d, respectively.

It is essential that no material intermixing of the gases in zones 34and 36 occur, or at least that it be confined to that region of thechamber adjacent to the bulkhead 9 l 24, where the work temperature isnot materially above that attained in the initial heat zone andtherefore still resistant to scaling in the atmosphere formed by theintermixing of small amounts of the atmosphere of zone 34 with that ofzone 36.

However, in order to prevent the flow of the atmosphere of zone 34 intozone 36, a number of precautions may be taken. The arrangement of thebulkhead 24 restricts such flow to a relatively shallow passage, andsince the rich atmosphere admitted beneath the bulkhead is of a somewhatlower temperature than that in zone 34, this richer gas has a greatertendency to occupy this level of the heating chamber than the hottermore oxidizing gas of zone 34, pressures in the two areas beingconsidered equal. Also, isolation of the oxidizing atmosphere to itszone 34 is assured by maintaining a pressure in zone 34 of a fewthousandths of an inch lower than that maintained in zone 36. It willalso be understood that a slight positive pressure, of the order of 0.01inch, is maintained in the entire heating chamber.

The relative pressures in the two zones is maintained by the controlledsuction effect of the nozzles 42a of zone 34 and nozzles 42b of zone 36.The amount of air required to complete the combustion of the gasesvented from zone 36 is fixed by the air-fuel ratio supplied to theburners 37 and the proportion of the reaction products to be withdrawnfrom zone 36 through the vents 40. With the volume of this air so fixed,the suctional effect produced thereby is determined by the adjustment ofthe nozzles 42b relative to the ports 43, so as to withdraw the desiredquantity of reaction products. The remainder of the reaction productspass from zone 36 either into zone 34 or zone 36, the combined ventingeffect of zone 36 being such as to maintain a positive pressure of about0.01 inch in this zone. The volume of air supplied to the nozzles 42a ofchamber 31 is approximately determined by the desired cooling of thecompletely combusted gases vented from zone 34 in order to bring thesegases to a sufficiently low temperature to permit them to be safelypassed through the heat exchanger 60. With the air supply so determined,the nozzles 42a are adjusted to also provide a pressure of about 0.01inch in zone 34. The slight pressure differential desired between zones36 and 34 is then automatically controlled by slightly increasing ordecreasing the air supply to the nozzles 42a to thereby increase ordecrease the suctional effect of this air on the flues 39. With thetubes 105 and 106 of pres sure regulator 104 connected to opposite sidesof the bulkhead 24, the diaphragm of the regulator 104 will be slightlyspring-biased so that the switch blade 112 will be on contact 103 whenthe pressure differential in zones 34 and 36 decreases below the desiredamount, contact 102 being-made when the pressure differential increasesabove this predetermined amount. Motor 101 will therefore rotate thevalve 100 to increase or decrease the air supply to the nozzles 42a insuch manner as to lower or raise the pressure in zone 34 to reestablishthe desired pressure relationship.

It is obvious that if it should be necessary to stop the movement ofwork through zone 34 for any appreciable period with a portion of theload still in this zone, such work might become undesirably scaled. Inorder to avoid scaling under such conditions the following procedure maybe employed. First, the entrance and exit doors and 21 are closed toisolate the furnace chamber from the external atmosphere. Thereafter,the switch 111 is moved to the left to energize valves 95a to 95d, 3595g, 97c, 97d and 9%. Valve 97:; closes down the oxidizing burners 35completely. Valves 95a, 95b, 95f and 95g place the zones 36 and 36 onreduced air and fuel supply, and valves 97c and 97d, which onlypartially close, reduce the air supply to the nozzles 42b and 420 belowthat normally supplied by by-passes 96c and 96d thereby decreasing thenormal on-control suction of these nozzles whereupon the pressure inzones 36 and 36 l a l 10 tends to build up. With no. gas being enteredinto zone 34 by its burners 35, the pressure in this chamber tends todecrease. Consequently the non-scaling atmosphere flows from zone 36 tozone 34 gradually flushing out and replacing the oxidizing atmosphere.This rich atmosphere in zone 34 will be burned in the arch chamber 31 bythe air supplied by nozzles 42a and to insure that these combustiongases will not exceed the safe operating temperature of the heatexchanger 60, a surplus of air may be fed to the nozzles at this time bya by-pass 113 disposed around the motor operated valve 100 andcontaining a normally closed electric valve 114 in circuit with themanual switch 111. The pressure differential maintaining mechanism isdisabled at this time by interruption of the power to motor 101 by themanual switch 111.

When it is desired to continue passage of work through the furnace,switch 111 is returned to the right restoring the furnace control tonormal.

Under the above procedure the entire furnace is supplied with anon-scaling atmosphere under low fire and, therefore, the load may beleft in the furnace during nonproductive periods such as noonhours orovernight. By adjusting the fuel supplied during this idling period toan amount just sufiicient to overcome furnace radiation losses, the workmay be kept substantially at final temperature during these idlingperiods whereby little or no time is lost in converting from the idlingto the productive cycle. Since the heating cycle for large billets maybe as long as three hours, this is an important saving in time.

In Fig. 8 the burners 37 and 38 are shown under the control of thesingle pyrometer 109. It is to be understood, however, that theseburners may be arranged in groups, each group being provided with airand fuel supply valve-s, such as a, 95b, 95 and 95g, controlled by aseparate pyrometer.

Obviously, numerous other modifications of the apparatus and the methodof operation will occur to those skilled in the art without departingfrom the essential principles of the invention.

prising a heating chamber having an inlet opening at one end for theadmission of work and an outlet opening at the opposite end for theremoval of Work, a partial barrier extending transversely across saidchamber to form an initial heating zone and a succeeding heating zonehaving a restricted passageway therebetween only sufiicient for theconvenient passage of work from the initial zone to the succeeding zone,burners extending into said chamber in each of said-zones, means forsupplying fuel and air to the burners of said initial heating zonehaving a ratio to produce combustion products which are scaling to metalabove a predetermined temperature, separate means for supplying fuel andair to the burners of the succeeding zone in a ratio to produce reactionproducts which are non-scaling to metal above said predeterminedtemperature, means responsive to the surface temperature of metalpassing through said initial heating zone for controlling the supply offuel and air to such zone to thereby restrict said surface temperatureto a value below said predetermined temperature, separate venting meansfor each of said zones and means for controlling the relative rate ofventing through said separate venting means.

2. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening at one end for the admission of Work andan outlet opening at the opposite end for the removal of work, a partialbarrier extending transversely across said chamber to form an initialheating zone and a succeeding heating zone having -a restrictedpassageway therebetween only sufficient for the convenient passage ofwork from the initial zone to the succeeding zone, burners extendinginto said chamber in each of said zones, means for supplying fuel andair to the burners of said initial heating zone having a ratio toproduce combustion products which are scaling to metal above apredetermined temperature, separate means for supplying fuel and air tothe burners of the succeeding zone in a ratio to produce reactionproducts which are non-scaling to metal above said predeterminedtemperature, means responsive to the surface temperature of metalpassing through said initial heating zone for controlling the supply offuel and air to such zone to thereby restrict said surface temperatureto a value below said predetermined temperature, venting means for saidsucceeding heating zone independent of said initial heating zone, andmeans for precluding passage of combustion products from said initialheating zone into the succeeding heating zone.

3. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening at one end for the admission of work andan outlet opening at the opposite end for the removal of work, a partialbarrier extending transversely across said chamber to form an initialheating zone and a succeeding heating zone having a restrictedpassageway therebetween only sufficient for the convenient passage ofwork from the initial zone to the succeeding zone, burners extendinginto said chamber in each of said zones, means for supplying fuel andair to the burners of said initial heating zone having a ratio toproduce combustion products which are scaling to metal above apredetermined temperature, separate means for supplying fuel and air tothe burners of the succeeding zone in a ratio to produce reactionproducts which are non-scaling to metal above said predeterminedtemperature, means responsive to the surface temperature of metalpassing through said initial heating zone for controlling the supply offuel and air to such zone to thereby restrict said surface temperatureto a value below said predetermined temperature, means for maintaining apredetermined positive pressure in said heating chamber, and means formaintaining the pressure in said initial heating zone at a valueslightly below the pressure maintained in the succeeding heating zone,said last means including separate venting means for each of saidchambers.

4. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening at one end for the admission of work andan outlet opening at the opposite end for the removal of Work, a partialbarrier extending transversely across said chamber to form an initialheating zone and a succeeding heating zone having a restrictedpassageway therebetween for the passage of work from the initial zone tothe succeeding zone, burners extending into said chamber in each of saidzones, means for supplying fuel and air to the burners of said initialheating zone having a ratio to produce combustion products which arescaling to metal above a predetermined temperature, separate means forsupplying fuel and air to the burners of the succeeding zone in a ratioto produce reaction products which are nonscaling to metal above saidpredetermined temperature, means responsive to the surface temperatureof metal passing through said initial heating zone for controlling thesupply of fuel and air to such zone to thereby restrict said surfacetemperature to a value below said predetermined temperature, independentvents for each of said zones, suction producing means in said vents, andmeans for controlling the suction means to cause a flow of reactionproducts into the initial heating zone from the succeeding heating zone.

5. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening for the admission of work to be heated,an outlet opening for the discharge of work, means dividing said chamberinto a plurality of intercommunicating successive heating zonesincluding an initial heating zone and a final heating zone, burnersextending into said chamber in each of said zones, means for supplying amixture of air and 12 fuel to the burners of said initial heating zonehaving a ratio to produce substantially complete combustion,

7 separate means for supplying air and fuel to the burners of eachsucceeding zone having a large deficiency of air, means for venting thecombustion products from said initial heating zone, separate means forventing the partially combusted products from each successive heatingzone, a combustion chamber disposed in heat transfer relation to saidfinal heating zone, said last mentioned venting means being incommunication with said combustion chamber for passage of said partiallycombusted products thereinto, and means for supplying air to saidcombustion chamber to produce further combustion of said partiallycombusted products in said combustion chamber.

6. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening for the admission of work to be heated,an outlet opening for the discharge of work, means dividing said chamberinto a plurality of intercommunicating successive heating zonesincluding an initial heating zone and a final heating zone, burnersextending into said chamber in said initial heating zone, means forsupplying a mixture of air and fuel to said burners having a ratio toproduce substantially complete combustion, separate means for supplyingto each succeeding zone hot combustion products of a mixture of fuel andair having a large deficiency of air, means for venting the combustionproducts from said initial heating zone, separate means for venting thepartially combusted products from each successive heating zone, acombustion chamber disposed in heat transfer relation to said finalheating zone, said last mentioned venting means being in communicationwith said combustion chamber for passage of said partially combustedproducts thereinto, and means for supplying air to said combustionchamber to produce further combustion of said partially combustedproducts in said combustion chamber.

7. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening for the admission of work to be heated,an outlet opening for the discharge of work, means dividing said chamberinto a plurality of intercommunicating successive heating zonesincluding an initial heating zone and a final heating zone, burnersextending into said chamber in each of said zones, means for supplying amixture of air and fuel to the burners of said initial heating zonehaving a ratio to produce substantially complete combustion, separatemeans for supplying air and fuel to the burners of each succeeding zonehaving a large deficiency of air, means for venting the combustionproducts from said initial heating zone, separate means for venting thepartially combusted products from each successive heating zone, acombustion chamber disposed in heat transfer relation to said finalheating zone, said last mentioned venting means being in communicationwith said combustion chamber for passage of said partially combustedproducts thereinto, means for supplying air to said combustion chamberto produce further combustion of said partially combusted products insaid combustion chamber, and means for maintaining a pressuredifferential between said initial heating zone and the next succeedingheating zone.

8. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening for the admission of work to be heated,an outlet opening for the discharge of work, means dividing said chamberinto a plurality of intercommunicating successive heating zonesincluding an initial heating zone and a final heating zone, burnersextending into said chamber in said initial heating zone, means forsupplying a mixture of air and fuel to said burners having a ratio toproduce substantially complete combustion, separate means for supplyingto each succeeding zone hot combustion products of a mixture of fuel andair having a large deficiency of air, means 13 7 v for venting thecombustion products from said initial heating zone, separate means forventing the partially combusted products from each successive heatingzone, a combustion chamber disposed in heat transfer relation to saidfinal heating zone, said last mentioned venting means being incommunication with said combustion chamber for passage of said partiallycombusted products thereinto, means for supplying air to said combustionchamber to produce further combustion of said partially combustedproducts in said combustion chamber, and pressure responsive meansassociated with the initial heating zone and the next successive heatingzone for causing flow of partially combusted products from the latterzone to the former.

9. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening for the admission of work, an outletopening for the discharge of work, means dividing said chamber into aplurality of intercommunicating successive heating zones including aninitial heating zone, an intermediate heating zone and a final heatingzone, burners extending into said initial heating zone, means forsupplying a mixture of fuel and air to said burners having a ratio toproduce substantially complete combustion, means for supplying anatmosphere to said intermediate heating zone composed of hot combustionproducts of fuel and air having a deficiency of air between 50% and 60%,separate means for supplying an atmosphere to said final heating chambercomposed of hot combustion products of fuel and air having adeficiencyof air between 40% and 50% and separate venting means for the initialand final heating zones whereby the atmosphere in said intermediateheating zone will be drawn in part into said initial heating zone and inpart into said final heating zone.

10. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening for the admission of work, an outletopening for the discharge of work, means dividing said chamber into aplurality of intercommunicating successive heating zones including aninitial heating zone, an intermediate heating zone and a final heatingzone, burners extending into said initial heating zone, means forsupplying a mixture of fuel and air to said burners having a ratio toproduce substantially complete combustion, means for supplying anatmosphere to said intermediate heating zone composed of hot combustionproducts of fuel and air having a deficiency of air between 50% and 60%,separate means for supplying an atmosphere to said final heating chambercomposed of hot combustion products of fuel and air having a deficiencyof air between 40% and 50%, separate venting means for the initial andfinal heating zones whereby the atmosphere in said intermediate heatingzone will be drawn in part into said initial heating zone and in partinto said final heating zone, and draft control means for each of saidventing means.

11. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening for the admission of work, an outletopening for the discharge of work, means dividing said chamber into aplurality of intercommunicating successive heating zones including aninitial heating zone, an intermediate heating zone and a final heatingzone, burners extending into said initial heating zone, means forsupplying a mixture of fuel and air to said burners having a ratio toproduce substantially complete combustion, means for supplying anatmosphere to said intermediate heating zone composed of hot combustionproducts of fuel and air having a deficiency of air between 50% and 60%,separate means for supplying an atmosphere to said final heating chambercomposed of hot combustion products of fuel and air having a deficiencyof air between 40% and 50%, venting means for the final heating zonewhereby the atmosphere in said intermediate heating zone will be drawnat least in part into said final heating zone, a combustion chamberdisposed in heat transfer relationship to I4- said final heating zone,said venting means communicating with said combustion chamber forpassage of the air deficient combustion products from said final heatingzone into said combustion chamber, and means for supplying air to saidcombustion chamber to produce further combustion of said productstherein.

12. A furnace for the scale-free heating of metal comprising-aheatingc'hamber having an inlet opening for the admission of work, anoutlet opening for the discharge of work, means dividing said chamberinto a plurality of intercommunicating successive heating zonesincluding an initial heating zone, an intermediate heating zone and afinal heating zone, burners extending into said initial heating zone,means for supplying a mixture of fuel and air to said burners having aratio to produce substantially complete combustion, means for supplyingan atmosphere to said intermediate heating zone composed of hotcombustion products of fuel and air having a deficiency of air between50% and 60%, separate means for supplying an atmosphere to said finalheating chamber composed of hot combustion products of fuel and airhaving a deficiency of air between 40% and 50%, venting means for thefinal heating zone whereby the atmosphere in said intermediate heatingzone will be drawn at least in part into said final heating zone, acombustion chamber disposed in heat transfer relationship to said finalheating zone, said venting means communicating with said combustionchamber, and means including an air jet for inducing a flow of airdeficient combustion products through said vent from said final heatingchamber into said combustion chamber and serving to support combustionthereof in said combustion chamber.

13. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening for the admission of work to be heated,an outlet opening for the discharge of work, means for dividing thechamber into a plurality of successive heating zones including aninitial heating zone and a final heating zone, burners extending intosaid chamber in said initial heating zone, means for supplying a mixtureof fuel and air to said burners having a ratio to produce substantiallycomplete combustion, separate means for supplying to each succeedingzone hot reaction products of a mixture of fuel and air having an airdeficiency of the order of 50%, means for venting said final heatingzone independently of said initial heating zone, means for venting saidinitial heating zone independently of said final heating zone, and meansfor concurrently interrupting the supply of fuel and air to said burnersand venting said final heating zone, at least in part, through saidinitial heating zone.

14. A furnace for the scale-free heating of metal comprising a heatingchamber having an inlet opening for the admission of work to be heated,an outlet opening for the discharge of work, means for dividing thechamber into a plurality of successive heating zones including aninitial heating zone and a final heating zone, burners extending intosaid chamber in said initial heating zone, means for supplying a mixtureof fuel and air to said burners having a ratio to produce substantiallycomplete combustion, separate means for supplying to each succeedingzone hot reaction products of a mixture of fuel and air having an airdeficiency of the order of 50%, means for venting said final heatingzone independently of said initial heating zone, means for venting saidinitial heating zone independently of said final heating zone, suctionproducing means in each of said venting means, and means forinterrupting the supply of fuel and air to said burners and controllingsaid suction means to vent said final heating zone, at least in part,through said initial heating zone.

15. A furnace for the scale-free heating of metal comprising a workheating chamber, means for supplying air and fuel to said chamber withan excess of fuel for complete combustion of the order of a combustionchamber in heat transfer relation to said heating chamber, means forventing said heatingchamber into said combustion chamber, means forsupplying air to said combustion chamber'for reaction therein with, saidvented products, a heat exchanger, means for venting the combustionproducts from said combustion chamber through said heat exchanger, meansdisposed between said combustion chamber and said heat exchanger foradding air to the vented combustion products tc control the temperaturethereof, and means for passing air through said heat exchanger out ofcontact with said combustion gases, said last means comprising saidmeans for supplying air to said work heating chamber.

1.6. A furnace for the scale-free heating of metal comprising a workheating chamber, means for supplying air and fuel to said chamber withan excess of fuel for complete combustion of the order of 100%, acombustion chamber in heat transfer relation to said heating chamber,means for venting said heating chamber into said combustion chamber,means for supplying air to said combustion chamber for reaction thereinwith said vented products, means for venting the combustion productsfrom said combustion chamber, means for cooling said combustion productsand means for passing said cooled combustion products in heat transferrelation to said means for supplying air to said heating chamber.

17. A furnace for the scale-free heating of metal comprising a heatingchamber, a restricted passageway extending through a wall of saidchamber for the passage of work, means for supplying an atmosphere forsaid furnace having a large deficiency in air for complete combustion,means for reducing combustion of said atmosphere with external air atsaid passageway including a vent in said passageway, suction means forsaid vent for producing a zone of reduced pressure in said passageway,said suction means including an air jet, a combustion chamber, and apassageway extending from said vent to said combustion chamber wherebythe atmosphere withdrawn through said vent will be combusted with theair introduced by said jet.

References Cited in the file of this patent UNITED STATES PATENTS1,332,684 Renner Mar. 2, 1920 1,406,424 Sorensen Feb. 14, 1922 2,078,747Vial Apr. 27, 1937 2,160,610 Witting May 30, 1939 2,233,474 DreffeinMar. 4, 1941 2,499,624 Bergstrom et a1. Mar. 7, 1950 2,499,704 Utterbacket a1. Mar. 7, 1950 2,620,174 Passafaro Dec. 2, 1952 FOREIGN PATENTS987,233 France Apr. 11, 1951 167,413 Germany Jan. 10, 1951

